1. Field of the Invention
The present invention relates to a method of injecting a liquid crystal material, which is capable of effectively and reliably injecting a liquid crystal material (such as smectic liquid crystal material) which so far could be injected with great difficulty or which could not be substantially injected at room temperature, and to an apparatus for injecting the liquid crystal material.
2. Related Background Art
Recent emerging development of liquid crystal display (LCD) devices for the TV application is outstanding. On the other hand, this new application of LCDs for TV, at the same time, requires a higher display performance than that which has ever been used in the field of LCDs.
For example, highly viscous liquid crystal materials for providing smectic liquid crystals can potentially realize a high image quality which is required for the TV application. However, due to the high viscosity of the smectic liquid crystal materials, the filling of a liquid crystal material to a panel comprising a pair of substrates (in particular, to a large screen TV panel) has still some critical problems. Although the ODF (One Drop Filling) technique is being used for filling large screen panels with conventional nematic liquid crystal materials, it is extremely difficult to fill a panel with a highly viscous smectic liquid crystal material by using the ODF technique. Accordingly, the filling of such a highly viscous smectic liquid crystal material needs an innovative filling method, in order to realize a practically effective liquid crystal material filing method for the smectic liquid crystal materials.
More specifically, the recent rapid development of the liquid crystal display technology has enabled the application of the LCDs to large screen TVs. This development has also been applied to large computer monitors such as 15-inch, 17-inch and over 20-inch diagonal screens. This rapid increase in the screen size has requested a new liquid crystal manufacturing method. Particularly, it is inevitable to improve the liquid crystal filling method. The conventional liquid crystal filling method that is known as the pressure deference method utilizing a pressure deference between vacuum and standard atmosphere consumes a lot of excess amount of liquid crystal, in particular for large panels. Moreover, the pressure difference method takes long time to fill large panels, which makes manufacturing throughput very low.
The ODF method introduced for large panel fillings requires minimum amount of liquid crystal material and much shorter filling time than conventional pressure difference method. Therefore, the ODF method is more popular than ever, in particular for the method of filling a large screen panel (such as one having a size of 30 inches or more).
More specifically, in the conventional method, a liquid crystal material is injected to an injection hole, which is preliminarily provided in a pre-bonded glass substrates. On the other hand, in this ODF methods a liquid crystal material is dropped on a lower glass substrate before the bonding, and then this lower glass substrate is covered with an upper glass substrate so as to provide a bonded glass substrates in one step. It is said that the number of the manufacturing steps and the gap (cell gap) between the glass substrates can be reduced by adopting such an ODF method. Particularly, in the case of twisted nematic (TN) LCDs, which are most popular in the LCD fabricating industry at present, the reduction in the cell gap can have a most significant effect on an increase in the decay time (or on a decrease in the liquid crystal response time).
On the other hand, requirement for large panel screen in LCD-TVs needs higher performance liquid crystal display mode than that of widely used TN (Twisted Nematic) LCDs. TN-LCDs have significant limitation in their optical response time and viewing angle, which are most required for TV or large-size TV image quality.
In order to overcome requirement for TV image quality, several nematic liquid crystal based LCD modes are being developed as well as smectic liquid crystal based LCD modes. Particularly, a smectic liquid crystal display is expected to be one of the most promising technologies to meet with the response speed, which is the last unsolved technical problem in the LCDs.
However, a smectic liquid crystal material generally has a very high viscosity such as that of wax-like material, it is almost impossible to apply the ODF method to the smectic liquid crystal materials. It is highly requested to establish innovative filling method which enables highly viscous smectic liquid crystal materials to fill large screen panels with effective manufacturing throughput, because the smectic liquid crystal material can provide a high-speed response suitable for large LCDs, particularly TVs.
In general, the following two liquid crystal filling methods are well known for a large screen panel manufacturing.                (1) Pressure difference method        (2) ODF method        
In the pressure difference method, a liquid crystal panel and liquid crystal material are set in the vacuum chamber. Air in the liquid crystal panel is sack up, then, the fill hole of the liquid crystal panel is touched with liquid crystal material, resulting in being covered by liquid crystal material. After the fill hole is covered by the liquid crystal material, the pressure in the vacuum chamber is gradually recovered to the atmospheric pressure. When the pressure in the vacuum chamber is recovered to the atmospheric pressure, the liquid crystal material is pushed into the interior of the panel due to the pressure difference between the interior of the liquid crystal panel (reduced pressure) and the chamber.
In the ODF method, a pre-formed perimeter seal pattern is provided on one of the two glass substrates, before the glass substrates are formed into a panel. A precisely measured amount of a liquid crystal material is dropped on the substrate having thereon the pre-formed perimeter seal pattern. Then, the other substrate of the two glass substrates is laminated on the above substrate to complete panel fabrication.
It is clear that the ODF method is much more effective than the pressure difference method in terms of volume manufacturing. Because of its liquid crystal dropping method, the ODF method is very effective for low viscous nematic liquid crystal materials. The dropped liquid crystal material on the pre-formed perimeter seal substrate is easily propagated to all over the substrate by the given pressure from laminated the other substrate.
On the contrary, highly viscous smectic liquid crystal material is not easy to propagate to all over the panel by the lamination pressure due to its high viscosity. Elevated temperature helps to reduce viscosity of the smectic liquid crystal materials, and makes uniform propagation to all over the substrate.
One of the problems of this temperature increase is volume expansion of materials. At the isotropic temperature such as 100° C., the viscous smectic liquid crystal material at room temperature shows low viscosity. This low viscosity effectively spreads out the liquid crystal material to all over the panel. After the liquid crystal is filled at the high temperature, the liquid crystal material is filled to all over the panel whose volume is expanded by high temperature. However, when the temperature is recovered to the room temperature, the volume of the liquid crystal material is decreased, so as to create bubbles in the panel.
Accordingly, when the temperature is elevated so as to provide a low viscosity of the liquid crystal material, such a method is not effective for the production of LCD panels.
As described above, it is eagerly desired to establish a method of injecting a liquid crystal material, which can sufficiently spread the high-viscosity liquid crystal material over the interior of a liquid crystal panel, and causes substantially no problem such as the creation of bubbles in the panel.